Phthalic anhydride is a useful intermediate chemical for making plasticizers, polyester resins and alkyd resins. It is commercially produced by a low pressure vapor-phase air oxidation process known as the von Heyden process. In this conventional process, either o-xylene or naphthalene is oxidized over a vanadium pentoxide/titanium dioxide catalyst contained in fixed bed reactors. Crude phthalic anhydride is recovered from the reactor effluent primarily as a solid by condensing it in multiple switch condensers.
There is a problem in the above-described conventional process relating to the recovery of the crude phthalic anhydride primarily as a solid in the multiple switch condensers. In particular, the problem is that switch condensers are expensive to build (as much as 20-30% of the total capital cost of the process is in the switch condenser section) and, due to solid dusting and plugging in the tubes, troublesome to operate and maintain.
Most of the prior suggestions to remedy the above problem focus on improving the switch condenser designs. U.S. Pat. No. 2,076,033 may be the earliest patent which discusses an improved switch condenser design. Other suggestions focused on eliminating the switch condenser entirely and include: continuous condensation and collection of the phthalic anhydride as a dust (U.S. Pat. No. 2,064,468) or as a slurry (U.S. Pat. No. 2,555,287); scrubbing the gas with a solvent, e.g., dibutyl phthalate (U.S. Pat. No. 2,574,644); using a moving bed of pebbles (U.S. Pat. No. 2,702,091); direct contact with a liquid coolant such as C.sub.n H.sub.2n+1 (U.S. Pat. No. 3,040,059); cooling the gas by vaporization of naphthalene (U.S. Pat. No. 3,112,324); finally compressing the cooled gas to 2-6 atmospheres and re-cooling it to recover the phthalic anhydride as liquid (German patent 1,158,051). None of these prior suggestions are commercially practiced today however. Indeed, switch condensers are still used exclusively in the present day phthalic anhydride processes and the industry continues to bear the high cost of the switch condenser and the problems associated with its operations and maintenance.
It is an object of the present invention to eliminate the problem of the conventional phthalic anhydride process relating to the recovery of the crude phthalic anhydride as a solid in the multiple switch condensers.